This invention relates to expanding gate valves and more particularly to an expanding gate valve provided with a hydraulically choked pneumatic-powered actuator.
In addition to or in place of manual means such as a handwheel, fluid-powered actuators are sometimes used for opening or closing valves. With single stem valves which handle very high pressures, the tremendous forces needed to operate the valve preclude the use of any manual means and require the provision of an automatic operator. Such automatic operators are also widely used for operating a valve in automatic response to sensing of some charged condition as, for example, with safety valves which are incorporated in safety systems.
Hydraulic actuators are generally preferred for subsea applications and where power, small size and reliability are important criteria. However, pneumatic actuators are oftentimes used where a reduction of the flammability hazard is important, where quicker response times are needed, and where pneumatic power is more readily available.
Heretofore, there has been little use of fluid-powered actuators with gate valves of the expanding gate type largely because of the associated problems. Variances in the sealing forces between expanding gate valves, even those of the same kind, usually rules out the adaptability of an actuator with a uniform thrust force for a plurality of valves wherein such variant characteristics are the general rule. INdeed, after repeated use a single valve will change its characteristics such that an actuator adjustment would be necessary. Also, the very large sealing forces of these valves require a very powerful actuator thrust to break the seal and such a powerful thrust can seriously damage the valve components, such as the seal rings or the gate and segment members. Very large springs also need be employed to return the actuator piston upon release of fluid pressure to the actuator and these significantly increase the size, weight and expense of the actuator. Accordingly, gate valves equipped with fluid-powered actuators have almost always been of the slab gate type where valve damage is less likely. Nevertheless, the handling of very high fluid pressures of 20,000 p.s.i. or more, as is characteristic of deep gas and oil wells, has made it very desirable that only gate valves of the expanding gate type be used, and particularly so in safety valves for very high pressure systems. Expanding gate valves, for example, provide an upstream seal which makes it possible to repair the valve while in operation.
Accordingly, it is an object of the invention to provide a valve of the expansible gate type having a pneumatic-powered actuator for opening and closing the valve.
It is another object to provide an expanding gate valve with a hydraulically choked pneumatic-powered actuator which is adapted for opening and closing the valve with reliability and safety.
It is still another object to provide an expanding gate valve having a pneumatic-powered actuator for opening and closing the valve which includes a readily adjustable means for controlling the stroke velocity of the actuator.
It is a further object to provide an expanding gate valve having a pneumatic-powered piston actuator for opening and closing the valve which can be readily converted to a double acting piston actuator to facilitate reciprocation of the piston.